The MGA With An Attitude
Finding TOP DEAD CENTER (TDC) Casually - CS-111A

There are lots of reasons and various occasions for needing to find Top Dead Center (TDC) for your engine. TDC is the point where a piston is exactly at the top of stroke, and the crankshaft connecting rod journal is exactly straight up (in a vertical engine of course). Sometimes the need is rather casual, like getting it in the ballpark so you can install the distributor drive gear in the correct orientation or install spark plug wires in the correct position. Other times it needs to be a bit more accurate, like setting the crankshaft straight up so you can do a blow down test without the crankshaft turning. There are also times when it has to be dead on accurate, like when installing a special camshaft and getting the non-standard cam timing exactly right. To this end I will explore a few ways to find TDC, starting with the easiest (and least accurate).

Very often you have a completely assembled engine, and you can use the timing mark on the crankshaft pulley to find TDC. This however does not tell you if it is on the compression stroke or exhaust stroke, which are 360 degrees apart, or offset by one complete rotation of the crankshaft. With TDC on the compression stroke the intake and exhaust valves are both closed. With TDC on the exhaust stroke the intake and exhaust valves are both open (partially open). For this you can remove the valve cover and look at the rocker arms to see which ones are fully up (slightly loose) and which ones are partly depressed (intake and exhaust arms at the same height), one moving up while the other is moving down. A quicker way is to remove spark plugs and place your thumb over #1 spark plug port, then rotate the crankshaft (clockwise is best but not essential). During transition between exhaust and intake strokes when one or both valves are open, there will be no pressure. During approach to TDC between compression and power strokes when both valves are closed, there will be pressure that will push your thumb off of the spark plug port to allow air to escape. When air stops blowing out it is fairly near TDC on the compression stroke. This may be close enough to install a distributor or spark wires, but not close enough for setting ignition timing or doing a blow down test. You would be lucky to get within 15 degrees of TDC using this method, so this is only practical for determining when the engine is on the compression stroke (or not).

Once the piston is close to TDC you might look into the spark plug port to see the piston move. You may also poke a small screwdriver into the port to feel the height of the piston. Caution: Never take your hand off of the screwdriver when it is in the spark plug port. If you turn the crankshaft with a tool in the cylinder, motion of the piston may damage the tool or the port threads or the piston. You can turn the crankshaft back and forth a little while observing motion of the piston to get a better idea of location of TDC, trying to estimate the position where the piston stops moving.

This is close, but not very accurate. There will be a small range of motion of the crankshaft near TDC that will produce very little motion of the piston. Crankshaft "throw" (eccentric distance) for the connecting rod journal in the MG (Austin) B-series engine is 1.75 inches (half of 3.5" stroke). Height of the piston is a function of cosine of the angle from TDC.  Cos(0)=1.000  Cos(5)=0.996   Change of height of the piston at 5 degrees will be (1.000-0.996)x1.75 = 0.007 inches. Therefore a change of 10 degrees at the crankshaft (+/-5d from TDC) only moves the piston 0.007", which is very hard to see or to feel with a probe. So sight or feel is not going to give you exact TDC position. This is not accurate enough for setting spark timing, but it may be close enough to do a blow down test.

If you have the valve cover off, you can inspect motion of the rocker arms. For a stock camshaft the intake and exhaust valve motion is mirror image. This means intake and exhaust valve will be open the same amount at TDC, and the rocker arms will be at the same height. You can lay a straight edge (steel rule or carpenters square or bubble level) across the two rocker arms, and it should be level. This will be significantly more accurate than the piston motion, because the ramp on the cam lobe is fairly steep at this point, which will induce more motion in the rocker arms. The relative motion is also doubled as one arm is moving up while the other is moving down, so this will move the straight edge out of level quickly with only a small movement of the crankshaft. By eye you may find TDC within 2 degrees. Using a bubble level you might find TDC within 1 degree (if the engine is standing level). This is close enough for checking if the timing mark on the crankshaft pulley is in the right place (consideration to harmonic balancers that may slip). It is also close enough for installation of the timing chain accurate to the nearest tooth on the sprockets. However (always one of these), this will not work with a non-standard camshaft where intake and exhaust valve timing is not mirror image.

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